The Huawei ME906 is yet another MDM9x15 based cellular modem that uses Linux on its Cortex-A5 processor, based on Qualcomm_Linux (LE). There are several variants (ME906A, ME906C, ME906J, ME906E, ME906V). There also is a ME906S-158, which is not a Qualcomm based device.

It seems like the Linux system is suspended after very few seconds of inactivity. To keep it alive, you can write any character to any of the serial devices. For example, the following one-liner script will work (assuming /dev/ttyUSB5 is one of the AT command ports of the modem):